Programming method to reduce gate coupling interference for non-volatile memory

ABSTRACT

A non-volatile memory device and programming process is described that compensates for coupling effects on threshold gate voltages of adjacent floating gate or non-conductive floating node memory cells by adjusting the threshold voltage level programmed in view of the data being programmed on a following programming cycle into adjacent memory cells, so that the coupling effect results in the desired target threshold voltages for the cells. In one embodiment of the present invention, memory cell coupling is compensated for by adjusting programming level of one or more memory cells of a first page a memory array to a higher or lower threshold verify target voltage given the data/programming level to be written to directly adjacent memory cells of a second page, so that coupling between the directly adjacent memory cells of the first and second pages brings the memory cells of first page to their final target programming level.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/142,293, filed Jun. 19, 2008, titled “PROGRAMMING METHOD TO REDUCEGATE COUPLING INTERFERENCE FOR NON-VOLATILE MEMORY” (allowed), which isa continuation of U.S. Pat. No. 7,400,532, issued Jul. 15, 2008, whichare commonly assigned and incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to memory devices and inparticular the present invention relates to EEPROM and Flash memorydevices.

BACKGROUND OF THE INVENTION

Memory devices are typically provided as internal storage areas in thecomputer. The term memory identifies data storage that comes in the formof integrated circuit chips. There are several different types of memoryused in modern electronics, one common type is RAM (random-accessmemory). RAM is characteristically found in use as main memory in acomputer environment. RAM refers to read and write memory; that is, youcan both write data into RAM and read data from RAM. This is in contrastto ROM, which permits you only to read data. Most RAM is volatile, whichmeans that it requires a steady flow of electricity to maintain itscontents. As soon as the power is turned off, whatever data was in RAMis lost.

Computers almost always contain a small amount of read-only memory (ROM)that holds instructions for starting up the computer. Unlike RAM, ROMcannot be written to. Memory devices that do not lose the data contentof their memory cells when power is removed are generally referred to asnon-volatile memories. An EEPROM (electrically erasable programmableread-only memory) is a special type non-volatile ROM that can be erasedby exposing it to an electrical charge. EEPROM comprise a large numberof memory cells having electrically isolated gates (floating gates).Data is stored in the memory cells in the form of charge on the floatinggates. A typical floating gate memory cell is fabricated in anintegrated circuit substrate and includes a source region and a drainregion that is spaced apart from the source region to form anintermediate channel region. A conductive floating gate, typically madeof doped polysilicon, or non-conductive charge trapping layer (afloating node), such as nitride (as would be utilized in asilicon-oxide-nitride-oxide-silicon or SONOS gate-insulator stack), isdisposed over the channel region and is electrically isolated from theother cell elements by a dielectric material, typically an oxide. Forexample, a tunnel oxide that is formed between the floating gate/nodeand the channel region. A control gate is located over the floatinggate/node and is typically made of doped polysilicon or metal. Thecontrol gate is electrically separated from the floating gate/node byanother dielectric layer. Thus, the floating gate or charge trappinglayer/floating node is “floating” in dielectric so that it is insulatedfrom both the channel and the control gate. Charge is transported to orremoved from the floating gate or trapping layer by specializedprogramming and erase operations, respectively, altering the thresholdvoltage of the device.

Yet another type of non-volatile memory is a Flash memory. A typicalFlash memory comprises a memory array, which includes a large number ofmemory cells. Each of the memory cells includes a floating gate orcharge trapping layer embedded in a field effect transistor (FET)transistor. The cells are usually grouped into sections called “eraseblocks.” Each of the cells within an erase block can be electricallyprogrammed by tunneling charges to its individual floating gate/node.Unlike programming operations, however, erase operations in Flashmemories typically erase the memory cells in bulk erase operations,wherein all floating gate/node memory cells in a selected erase blockare erased in a single operation. It is noted that in recent Flashmemory devices multiple bits have been stored in a single cell byutilizing multiple threshold levels or a non-conductive charge trappinglayer with the storing of data trapped in a charge near each of thesources/drains of the memory cell FET (such as in certain SONOS devicesor so called “NROM” devices).

A problem in programming non-volatile memory devices is that memorycells can be capacitively coupled during the programming of adjacentcells of the row or the programming of memory cells of adjacent wordlines or rows, interfering with or even program disturbing the coupledmemory cell and inadvertently changing its stored charge and thresholdvoltage level. This is particularly an issue in modern NAND architectureFlash memory devices where the smaller memory cell separation and cellpitch can increase this coupling effect. In addition, the effect of cellto cell coupling is likely to continue to worsen with futuremanufacturing process improvements that will further reduce featuresizes and cell separation and in Flash memory and other non-volatilememory devices that utilize low operating voltages single level cells(SLCs) or multi-level cells (MLCs) that have narrow logic separationwindows between their differing logic states.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art foralternative methods of programming Flash memory arrays.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a system containing anon-volatile memory device in accordance with an embodiment of thepresent invention.

FIGS. 2A and 2B are simplified block diagrams of NAND and NORarchitecture Flash memory arrays in accordance with an embodiment of thepresent invention.

FIGS. 3A and 3B detail programming operations in a non-volatile memoryarray in accordance with embodiments of the present invention.

FIG. 4 details programming operations in a non-volatile memory arrayhaving multiple pages per row in accordance with embodiments of thepresent invention.

FIGS. 5A-5C detail programming operations in a non-volatile memory arrayutilizing multi-level cells and having multiple pages per row inaccordance with embodiments of the present invention.

FIG. 6 details another programming operation in a non-volatile memoryarray utilizing multi-level cells and having multiple pages per row inaccordance with embodiments of the present invention.

FIG. 7 is a simplified block diagram of a memory module in accordancewith an embodiment of the present invention.

DETAILED DESCRIPTION

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings that form a part hereof,and in which is shown by way of illustration specific preferredembodiments in which the inventions may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention, and it is to be understood that otherembodiments may be utilized and that logical, mechanical and electricalchanges may be made without departing from the spirit and scope of thepresent invention. The terms wafer or substrate used in the followingdescription include any base semiconductor structure. Both are to beunderstood as including silicon-on-sapphire (SOS) technology,silicon-on-insulator (SOI) technology, thin film transistor (TFT)technology, doped and undoped semiconductors, epitaxial layers of asilicon supported by a base semiconductor structure, as well as othersemiconductor structures well known to one skilled in the art.Furthermore, when reference is made to a wafer or substrate in thefollowing description, previous process steps may have been utilized toform regions/junctions in the base semiconductor structure, and termswafer or substrate include the underlying layers containing suchregions/junctions. The following detailed description is, therefore, notto be taken in a limiting sense, and the scope of the present inventionis defined only by the claims.

Embodiments of the present invention include non-volatile memory devicesand arrays that compensate for the capacitive coupling effects onprogrammed threshold gate voltages of adjacent floating gate ornon-conductive floating node memory cells by adjusting the thresholdvoltage level programmed into a memory cell in view of the couplingratio and the data/threshold value being programmed on a followingprogramming cycle into an adjacent memory cell, so that the couplingeffect between the cells results in the desired target thresholdvoltages for the cells. In one embodiment of the present invention,memory cell to memory cell coupling is compensated for by adjustingprogramming level of a first memory cell given the data/programminglevel to be written in a second programming operation to an adjacentsecond memory cell of the memory array so that coupling between theadjacent memory cells will bring the first memory cell to its finaltargeted programming level for the data stored in first memory cell. Inanother embodiment of the present invention, memory cell to memory cellcoupling is compensated for by adjusting programming level of one ormore memory cells of a first page or row of a memory array ornon-volatile memory device to a higher or lower threshold verify targetvoltage given the data/programming level to be written to directlyadjacent memory cells of a second page, so that coupling between thedirectly adjacent memory cells of the first and second pages brings theone or more memory cells of first page to their final target programminglevel.

Common programming technique for non-volatile floating gate or floatingnode memories programs a bit or row (commonly referred to as a page) ofthe memory by applying a programming voltage or series of programmingvoltage pulses to the control gates of the selected memory cells andprogramming or inhibiting the selected memory cells to either program(set at logical “0” by the injection of charge to the floating gate orfloating node of a memory cell) or inhibit (not program, usuallyintended to leave the cell erased and set at logical “1”) by couplingtheir channels to either a program or inhibit voltage. As stated above,because of cell to cell coupling, the elevated programming voltagecoupled to the control gates of memory cells can have the undesiredeffect of altering the threshold voltages in adjacent memory cells. Thiscan cause threshold voltages to be moved out of their specified logicwindows, particularly in modern low voltage or multi-level cells, andthereby corrupt the stored data.

FIG. 1 details a simplified diagram of a system 128 incorporating anon-volatile memory device 100 of an embodiment of the present inventionconnected to a host 102, which is typically a processing device ormemory controller. The non-volatile memory 100, such as a Flash memorydevice, has a control interface 106 and an address/data interface 108that are each connected to the processing device 102 to allow memoryread and write accesses. It is noted that in alternative embodiments,the address/data interface 108 can be divided into separate interfaces.Internal to the non-volatile memory device a control statemachine/control circuit 110 directs the internal operation; managing thenon-volatile memory array 112 and updating RAM control registers anderase block management registers 114. The RAM control registers andtables 114 are utilized by the control state machine 110 duringoperation of the non-volatile memory 100. The non-volatile memory array112 contains a sequence of memory banks or segments 116, each bank 116is organized logically into a series of erase blocks (not shown).

Memory access addresses are received on the address/data interface 108of the non-volatile memory 100 and divided into a row and column addressportions. On a read access, the row address is latched and decoded byrow decode circuit 120, which selects and activates a row page (notshown) of memory cells. The bit values encoded in the output of theselected row of memory cells are connected from a local bitline/string(not shown) to a global bitline (not shown) and detected by senseamplifiers 122 associated with the memory bank. The sense amplifiers 122also typically include a data cache and write data latch circuits (notshown). The column address of the access is latched and decoded by thecolumn decode circuit 124. The output of the column decode circuitselects the desired column data from the sense amplifier outputs andconnected to the data buffer 126 for transfer from the memory devicethrough the address/data interface 108.

On a write access, the row decode circuit 120 selects the row page andcolumn decode circuit selects write sense amplifiers 122. Data values tobe written are connected from the data buffer 126 to the data cache andthen to the write data latches of the write sense amplifiers 122selected by the column decode circuit 124 and written to the selectednon-volatile memory cells (not shown) of the memory array 112. Thewritten cells are then reselected by the row and column decode circuits120, 124 and sense amplifiers 122 so that they can be read to verifythat the correct values have been programmed into the selected memorycells. It is noted that in one embodiment of the present invention, thecolumn decode 124 may be optionally placed between the memory array 112and the sense amplifiers 122.

Two common types of non-volatile or Flash memory array architectures arethe “NAND” and “NOR” architectures, so called for the resemblance whichthe basic memory cell configuration of each architecture has to a basicNAND or NOR gate circuit, respectively. In the NOR array architecture,the memory cells of the memory array are arranged in a matrix of rowsand columns similar to conventional RAM or ROM. The gates of eachnon-volatile memory cell of the array matrix are coupled by rows to wordselect lines (word lines or WLs) and their drains are coupled to columnbit lines. The source of each memory cell is typically coupled to acommon source line. The NOR architecture floating gate or floatingnode/charge trapping memory array is accessed by a row decoderactivating a row of memory cells by selecting the word line coupled totheir gates. The row of selected memory cells then place their storeddata values on the column bit lines by flowing a differing current fromthe coupled source line to the coupled column bit lines depending ontheir programmed states. A column page of bit lines is selected andsensed, and individual data words are selected from the sensed datawords from the column page and communicated from the memory.

A NAND memory array architecture also arranges its array of non-volatilememory cells in a matrix such that the gates of each memory cell of thearray are coupled by rows to word lines. However each memory cell is notdirectly coupled to a source line and a column bit line. Instead, thememory cells of the array are arranged together in strings, typically of8, 16, 32, or more each, where the memory cells in the string arecoupled together in series, source to drain, between a common sourceline and a column bit line. This allows a NAND array architecture tohave a higher memory cell density than a comparable NOR array, but withthe cost of a generally slower access rate, greater cell to cellcoupling, and programming complexity.

A NAND architecture floating gate or floating node memory array isaccessed by a row decoder activating a row of memory cells by selectingthe word select line coupled to their gates. In addition, the word linescoupled to the gates of the unselected memory cells of each string arealso driven. However, the unselected memory cells of each string aretypically driven by a higher gate voltage so as to operate them as passtransistors and allowing them to pass current in a manner that isunrestricted by their stored data values. Current then flows from thesource line to the column bit line through each floating gate/nodememory cell of the series coupled string, restricted only by the memorycells of each string that are selected to be read. This places thecurrent encoded stored data values of the row of selected memory cellson the column bit lines. A column page of bit lines is selected andsensed, and then individual data words are selected from the sensed datawords from the column page and communicated from the memory device.

It is noted that embodiments of the present invention are not limited toNAND or NOR architecture memory arrays or memory devices and can applyto other block erasing memory array architectures and memory devices,including, but not limited to, AND and virtual ground architecturememory arrays and memory devices.

FIGS. 2A and 2B show schematics of a simplified NOR and NANDarchitecture floating node or trapping layer memory array 200, 250 of aFlash memory device of an embodiment of the present invention. It isnoted that the memory arrays 200, 250 of FIGS. 2A and 2B are forillustrative purposes and should not be taken as limiting and that othermemory array embodiments of the present invention are possible and willbe apparent to those skilled in the art with the benefit of the presentdisclosure.

In FIG. 2A, a series of NAND memory strings 220 are arranged in an array200 and coupled to bit lines 212 and source lines 214. In each NANDmemory string 220, a series of floating gate or floating node memorycells 202 of embodiments of the present invention are coupled togethersource to drain to form the NAND string 220 (typically having 8, 16, 32,or more cells). As described above, each floating gate/node memory cellFET 202 has a gate-insulator stack formed over the channel region. Tofurther enable operation, in one embodiment of the present invention,each NAND architecture memory string 220 of the memory is formed in anisolation trench, allowing the substrate of each isolation trench to beindividually biased for programming and erasure. The word lines 206couple across the NAND strings 220, coupling the control gates ofadjacent memory cells 202 enabling a single memory cell 202 in eachmemory string 220 to be selected. In each NAND memory string 220,impurity (N+ typically) doped regions are formed between each gateinsulator stack to form the source and drain regions of the adjacentmemory cells 202, which additionally operate as connectors to couple thecells of the NAND string 220 together. In one embodiment of the presentinvention, the N+ doped regions are omitted and a single channel regionis formed under the NAND memory string 220, coupling the individualmemory cells 202. Each NAND memory string 220 is coupled to select gates204 that are formed at either end of each NAND string 220 andselectively couple opposite ends of each NAND string 220 to a bit line212 and a source line 214. The select gates 204 are each coupled to gateselect lines, select gate drain {SG(D)} 210 and select gate source{SG(S)} 208, that control the coupling of the NAND strings to the bitlines 212 and source lines 214, respectively, through the select gates204. In FIG. 2A, the substrate connection 222 is shown coupled to eachNAND string 220, allowing the memory cells 202 of each NAND string 220to be biased for erasure.

FIG. 2B details a simplified NOR floating gate/node memory array 250 ofa Flash memory device embodiment of the present invention. In FIG. 2B,floating gate/node memory cells 202 are coupled together in a NORarchitecture memory array having bit lines 212, source lines 214, wordlines 206, and substrate connection 222. Each floating gate memory cell202 has a floating gate or floating node/charge trapping field effecttransistor (FET) comprising a gate-insulator stack formed between drainand source regions that are coupled to a bit line 212 and a source line214, respectively.

As stated above, common programming technique for non-volatile memoriesor Flash memories programs a bit or page of data (where one or morepages can be contained in a row of a memory array) of the memory byapplying a programming voltage or series of programming voltage pulsesto the control gates of the selected memory cells and programming orinhibiting the selected memory cells to either program (set at logical“0” by the injection of charge to the floating gate or floating node ofa memory cell) or inhibit (not program, usually intended to leave thecell erased and set at logical “1”) by coupling their channels to eithera program or inhibit voltage.

In programming a page of memory cells in a NOR architecture Flash/EEPROMmemory array, a programming voltage is typically coupled to the wordlines coupled to the gates of the selected memory cells of the page. Thememory cells are also coupled to a bit line or source line that has achannel program or program-inhibit voltage placed upon it. This appliesa field across the page of memory cells selected for programming thatcauses carriers to tunnel to the floating gate or charge trapping layer,altering the threshold voltage level (Vt) of the selected memory celland storing the data value. If a program-inhibit voltage placed upon thecoupled bit lines or source lines, the applied field is not sufficientto tunnel carriers to the floating gate/nodes of the memory cells and nodata value is programmed, the memory cell remaining in the prior orerased state.

In programming a page of memory cells in a NAND architectureFlash/EEPROM memory array, a programming voltage is typically coupled toword lines coupled to the gates of the selected memory cells of thepage, each memory cell typically in a different memory string of thearray. In addition, the word lines coupled to the gates of theunselected memory cells of each string are also driven by a pass gatevoltage (Vpass) so as to operate them as pass transistors, generating achannel of carriers by capacitive coupling in the unselected memorycells and allowing them to pass current in a manner that is relativelyunrestricted by their stored data values. The memory strings are thentypically coupled to a bit line or source line that has a program orprogram-inhibit voltage placed upon it. As stated above, the appliedpass voltage, Vpass, also allows generation of a channel of carriers inthe memory string and allows the memory cell that was selected forprogramming in each string to couple through this channel of carriers tothe bit line or the source line. This applies a field across theselected memory cell of each string that causes carriers to tunnel tothe floating gate or charge trapping layer, altering the thresholdvoltage level (Vt) of the selected memory cell and storing the datavalue. If a program-inhibit voltage placed upon the coupled bit line orsource line, the applied field is not sufficient to tunnel carriers tothe floating gate/node, no data value is programmed and the memory cellremains in the erased state.

After programming the selected page of memory cells in both NAND and NORarchitecture Flash memory, a verify operation is then performed toconfirm that the data was successfully programmed. If one or more memorycells of the programmed page fails verification, the program and verifycycle is repeated until the data is successfully programmed or aselected number of iterations have passed and the programming operationis deemed to have failed.

It is noted that a programming cycle is also utilized in eraseoperations in block based non-volatile memories. In block erasure, theblock selected for erasure is typically erased with a series of highvoltage erase pulses. The contents of the newly erased block are thenread to verify that they were completely erased and soft programmed to“heal” any overerased memory cells. Data comparison is used during theverification portion of this erase cycle to confirm that the memorycells of each row of the erase block has been completely erased and thatno further erase pulses are required. Verification is then typicallyused to locate and mark any bad rows or sectors of the array so thatthey are not used.

A page of data is typically defined as the number of columns of datathat can be read from the memory array with a single open pageoperation. Pages are usually sequentially addressed within the array.The page open operation is generally accomplished with the initial“ACTIVE” command of the two command sequence of a synchronous memoryread or write operation (or the RAS command in an asynchronous memory),which activates the selected row and couples the memory cells to thesense amplifiers to be read or written. The ACTIVE command is thenfollowed by a “READ” or “WRITE” command (or the CAS command combinedwith WE in an asynchronous memory) that selects the data word to be reador written to or from the columns of the open page or indicates theselected starting data word of a multi-word read or write burst.

As stated above, many modern non-volatile memory devices have one ormore pages of data contained on each row of the memory array. Intypically in these cases the array will also have multiple bit lines persense amplifier that are selectively coupled to the sense amplifierdepending on the page being opened. For example in a two page per rowarray each sense amplifier will have even and odd bit lines that runside by side through the array and are selectively coupled to the arraydepending on whether the even or odd numbered page of the row of thearray is selected.

Programming and reading pages within an array is typically done in asequential fashion, with a lower addressed page generally being read orwritten before a higher addressed page. In programming and readingmulti-page per row, since there are multiple pages per row, the pagesare generally programmed or read from lowest addressed to highest withinthe row, the same pattern is repeated on higher addressed rows asrequired.

As stated above, a problem in programming non-volatile memory devices isthat memory cells can capacitively couple to each other during theprogramming of adjacent cells of the row or the programming of memorycells of adjacent word lines or rows, program disturbing or interferingwith the coupled memory cell and inadvertently changing its storedcharge and threshold voltage level. This is particularly an issue inmodern NAND architecture Flash memory devices where the smaller memorycell separation and cell pitch can increase this coupling effect. Thecoupling effects are typically highest in memory cells that are directlyadjacent to each other within the row or column (X-axis or Y-axis)orientation of the memory array. Coupling effects are also an issue inFlash memory and other non-volatile memory devices that utilize reducedfeature size, low operating voltage single level cells (SLCs) ormulti-level cells (MLCs) that utilize a narrow logic separation windowof programmed threshold voltage between different logic states.

In programming memory cells, target threshold voltage ranges withindefined “logic windows” are utilized to define differing stored logicstates in the floating gate or floating node memory cell. When a page ofmemory cells are programmed or erased the threshold voltage levels ofthe cells of the page are altered in iterating programming and verifycycles (or erase and verify cycles) until the threshold voltages of thememory cells verify within the desired logic window for the data state.Typically, in multi-level cells (MLCs), a series of different thresholdvoltage level (Vth) ranges/logic windows are defined to indicate thediffering stored data states within the cell (such as, a different Vthrange for each of 00, 01, 10, and 11 bit states for a two data bit MLC).Program disturb or interference occurs when the threshold voltage levelof a cell is pushed outside the Vth range/logic window for its storeddata value by the coupled programming voltage being applied to anadjacent memory cell/word line. Because of this, as noted above, programdisturb or interference is a particular issue with multi-level cells(MLCs) with their multiple Vth ranges/logic windows.

As stated above, embodiments of the present invention compensate formemory cell to memory cell coupling in order to reduce the impact onstored data values by adjusting programming level of a first memory cellgiven the data/programming level to be written at a later time to secondadjacent memory cell of the memory array so that coupling between theadjacent memory cells will bring the first memory cell to its finaltargeted programming level for the data stored in first memory cell. Inone embodiment, the capacitive coupling effects between adjacentfloating gate or non-conductive floating node memory cells iscompensated for by programming a first memory cell to a lower thresholdgate voltage if a second adjacent memory cell is also to be programmedso that, when the second memory cell is programmed, the threshold gatevoltage of the first memory cell is raised through coupling to be withinthe target threshold voltage range for the data that is stored in it. Inanother embodiment of the present invention, memory cell to memory cellcoupling is compensated for by adjusting programming level of one ormore memory cells of a first page or row of a memory array ornon-volatile memory device to a higher or lower threshold verify targetvoltage given the data/programming level to be written to directlyadjacent memory cells of a second page, so that coupling between thedirectly adjacent memory cells of the first and second pages brings theone or more memory cells of first page to their final target programminglevel. In yet another embodiment, the capacitive coupling effectsbetween adjacent multi-level floating gate/node memory cells (MLCs),that store two or more bits of data in a single memory cell through theuse of multiple threshold voltage levels or through charge storagecentroids on a non-conductive floating node is compensated for byprogramming a first memory cell to a lower threshold gate voltage if asecond adjacent memory cell is also to be programmed. In a furtherembodiment, in a non-volatile memory device having multiple pages ofmemory per row of the memory array (such as an array having even and oddbit lines running side by side through the array), memory cell to memorycell coupling is compensated for by adjusting programming level of oneor more memory cells of a first page of the memory array to a higher orlower threshold verify target voltage given the data/programming levelto be written to memory cells of a second page that is directly adjacentin a X (within the row) or Y (in the following row) orientation withinthe memory array, so that coupling between the directly adjacent memorycells of the first and second pages brings the one or more memory cellsof first page to their final target programming level.

FIGS. 3A-6 illustrate typical array programming operations andinterference compensation of various embodiments of the presentinvention. It is noted that although the programming operationsdescribed in FIGS. 3-6 are described in reference to NAND architecturememory arrays it noted that the descriptions are for illustrativepurposes and that the embodiments are applicable to the programming ofother non-volatile memory array architectures and should not be taken aslimiting.

FIGS. 3A and 3B illustrate an example of a programming operationutilizing coupling compensation in accordance with an embodiment of thepresent invention. In FIG. 3A, the threshold voltages (Vth) programmedinto a first row 306 of memory cells 302 coupled to word line WLN of aNAND architecture memory array 300 of an embodiment of the presentinvention are selected based on the data to be programmed into a secondrow 308 of memory cells 304 coupled to word line WLN+1. When the secondrow 308 of memory cells 304 is then programmed, the coupling betweenadjacent memory cells 302, 304 of the first and second rows 306, 308will adjust the threshold voltages of the programmed memory cells of thefirst row 306 to the final target threshold voltage values (within thelogic window) for data values stored.

As stated above, in non-volatile memory devices and arrays that utilizefloating gate or floating node memory cells, cells are typically erasedinto low threshold voltage level representing a logical “1” state and,when programmed, the threshold voltages of select memory cells of a pageand/or row of memory are altered by the injection of a charge into thefloating gate/node raising the threshold voltage to a logical “0” state.In a typical multi-level cell, the erased state is a logical “11” statethat is changed in one or two programming operations to a thresholdvoltage within logical windows representing a logical “10”, “10”, or“00” state when programmed. Interference of adjacent memory cellsgenerally occurs when a selected memory cell is programmed to alter itsthreshold voltage from the erased state and typically has the effect ofinadvertently injecting additional charge into the adjacent memory cellthat was not selected and altering its threshold voltage.

Compensating for this inadvertent charge injection in the adjacentmemory cell via coupling interference can be accomplished by loweringthe threshold voltage of the adjacent memory cell. Adjacent memory cellsthat are in the erased state can be altered to a lower erased statethreshold voltage during erasure. Adjacent memory cells that are in theprogrammed state can be programmed to a lower threshold voltage, whichis then adjusted upwards by the coupling interference. It is also notedhowever that in many cases the logical window for the erased statethreshold voltage is much larger than that of the programmed state andtherefore, generally more immune to disturb than the programmed state.The coupling ratio is typically dependent on the design and materials ofthe memory cells and the memory array and dictates the amount that thethreshold voltages of adjacent memory cells should be adjusted tocompensate. As such, the coupling ratio can be determined from thedesign and/or characterized during manufacture. Coupling ratiocharacterization of a memory is typically done on a design or “stepping”basis or, for a precise match, on an individual memory die. The couplingratio is also higher between memory cells of an array that are directlyadjacent to each other. In an array, this typically means that thecoupling is higher, and therefore coupling interference moresignificant, between memory cells that are directly adjacent to eachother in on the X-axis (cells adjacent within the row) or Y-axis (cellsadjacent within a column) directions and is less significant in cellsthat are displaced from each other or only adjacent on the diagonal.

FIG. 3B details the threshold voltage ranges 350 and the adjustment ofthe programmed state threshold voltages 354, 356 of the first row 306 ofsingle level memory cells (SLCs) 302, 304 of the memory array 300embodiment of FIG. 3A. In FIG. 3B, memory cells 302, 304 are initiallyerased into the erased threshold voltage state 352 (logical state “1”, aslightly negative voltage). When the row is programmed, the thresholdvoltages of a row 306 (or page) of memory cells 302 that are placed intothe programmed state (and not inhibited and left in an erased state) arealtered to either a higher (such as 0.8V) 356 or a lower (such as 0.6V)354 threshold voltage, depending on whether a directly adjacent (ornon-directly adjacent) memory cell 304 of the following row 308 is alsoto be altered to a programmed state on the following programmingoperation. When the adjacent memory cells 304 of the following row 308are then programmed at a later time, the memory cells 302 of the initialrow 304 will also be altered through coupling to the final target (suchas 0.8V, representing logical “0”) threshold voltage level.

FIG. 4 illustrates an example of a programming operation in anon-volatile memory array 400 having multiple pages per row utilizingcoupling compensation in accordance with an embodiment of the presentinvention. In FIG. 4, the array 400 contain non-volatile memory cells402 arranged in rows 404, 406, 408 and columns 410, 412, 414, 416 andhaving two pages (a page on the even bit lines 410, 414 and a page onthe odd bit lines 412, 416) in each row. As there are multiple pagescontained in each row, major coupling interference effects can occur inboth within the row (the X-axis) and in the column (from row to row, orY-axis), whereas in the embodiment of FIG. 3, with a single page perrow, the major coupling effects were mainly row to row (column directionor Y-axis). The page address/programming sequence of the memory arrayembodiment 400 of FIG. 4 transitions in an alternating “S” patternbetween the even bit line 410, 414 and the odd bit line 412, 416 so thatthe next memory cell/page to be sequentially addressed/programmed isalways directly adjacent to the previously addressed/programmed page(sequentially writing the pages of the row from the first page to thelast page and then reversing and writing the last to first page of thenext row, and so on). This minimizes circuit complexity, the amount ofrequired input buffer/latches space, and allows for ease of compensatingthe threshold voltage level of the current page as the data for the nextpage is generally already latched in the input buffer/data latch for thenext programming operation. For example, Page 0 420 (Even BL 410, 414,WLN 404), Page 1 422 (Odd BL 412, 416, WLN 404), Page 2 424 (Odd BL 412,416, WLN+1 406), Page 3 426 (Even BL 410, 414, WLN+1 406), Page 4 428(Even BL 410, 414, WLN+2 408), Page 5 430 (Odd BL 412, 416, WLN+2 408),etc. It is noted that in multiple page per row embodiments of thepresent invention coupling interference can also be compensated multiplewrite operations out/pages out, such as from page 0 420 to page 3 426(which is also directly adjacent in the alternating “S” programmingpattern, but are out of direct sequence as page 3 426 is programmed onthe return loop of the “S” while page 0 420 is programmed in the initialwrite operation). However, this requires increased circuit complexity(to compensate for multiple disturb events) and additional inputbuffer/data latch space. The additional input buffer/latches areutilized to hold the four future write operations/pages of data requiredto allow page 0 420 to be compensated for page 3's 426 data and page 2424 with page 5's 430.

In prior art memory arrays with multiple pages per row, the pageprogramming sequence is typically a reversed “Z” pattern, sequentiallywriting the pages of the row from the first page to the last page (evenbit lines/page 0 and odd bit lines/page 1 in FIG. 4) and then returningto write the first page of the next row. For example, Page 0 420 (EvenBL 410, 414, WLN 404), Page 1 422 (Odd BL 412, 416, WLN 404), Page 3 426(Even BL 410, 414, WLN+1 406), Page 2 424 (Odd BL 412, 416, WLN+1 406),Page 4 428 (Even BL 410, 414, WLN+2 408), Page 5 430 (Odd BL 412, 416,WLN+2 408), etc. It is also noted that, while such a prior artprogramming sequence is not optimal for coupling compensation, as writeoperations are sequenced so that that the next memory cell/page to besequentially addressed/programmed are not always directly adjacent tothe previously addressed/programmed page. However, this prior art pagewrite sequence can also be compensated for memory cell to memory cellcoupling utilizing embodiments of the present invention. It is furthernoted, however, that this coupling compensation in the prior artprogramming sequence, because of its pattern, would benefit fromcompensation utilizing additional input buffer/latches are utilized tohold the three future write operations/pages of data that would berequired to allow page 0 420 to be compensated for page 3's 426 data,page 1 412 with page 2's 424, etc.

FIG. 5A illustrates an example of a programming operation in amulti-level cell (MLC) non-volatile memory array 500 having multiplepages per row utilizing coupling compensation in accordance with anembodiment of the present invention. In FIG. 5, the array 500 containsMLC non-volatile memory cells 502 that store 2-bits of data per cellarranged in rows 504, 506, 508 and columns 510, 512 and having fourpages (two pages on each memory cell 502 on the even bit lines 510 andtwo pages on each memory cell 502 on the odd bit lines 512) in each row.The pages are written in sequential fashion, such that a first page iswritten into a group of memory cells of the row (writing the first bitinto each cell), before the threshold voltages of the group of memorycells are altered yet again to write a second page into the group ofmemory cells (writing the second bit into each cell) in two separatewrite operations, before moving on to another group of memory cells ofthe row to write a third and fourth pages of data, and so on. This writesequence is detailed in threshold voltage transitions 540 in FIG. 5B. InFIG. 5B, the lower page data 542 is written into a memory cell 502,programming the memory cell to the threshold voltage level representinglogic state “10” 546 or inhibiting the memory cell and leaving thememory in the erased logic state of “11” 544. The upper page data 548 isthen written into the memory cell 502. If the second page data islogical “0”, programming the memory cell to either the threshold voltagelevel representing logic state “00” 550 or the threshold voltage levelrepresenting logic state “01” 552 depending on if the stored first pagedata is logical state “11” or “10”. If the second page data is logical“1”, inhibiting the memory cell and leaving the memory in the erasedlogic state of “11” 544 or logic state “10” 546, again depending on ifthe stored first page data at logical state “11” or “10”. It is notedthat in another embodiment of the present invention the pages stored ina group of MLC memory cells are written into the memory cells in asingle write operation (writing the first and second bits into each cellat the same time).

As there are multiple pages contained in each row, major couplinginterference effects can occur in both within the row (the X-axis) andin the column (from row to row, or Y-axis), as with the SLC embodimentof FIG. 4. The page address/programming sequence of the memory arrayembodiment 500 of FIG. 5 transitions in an alternating “S” patternbetween the even bit line 510 and the odd bit line 512 so that the nextmemory cell to be sequentially addressed/programmed is always directlyadjacent to the previously addressed/programmed cell. For example, Pages0 and 1 520 (Even BL 510, WLN 504), Pages 2 and 3 522 (Odd BL 512, WLN504), Pages 4 and 5 524 (Odd BL 512, WLN+1 506), Pages 6 and 7 526 (EvenBL 510, WLN+1 506), Pages 8 and 9 528 (Even BL 510, WLN+2 508), Pages 10and 11 530 (Odd BL 512, WLN+2 508), etc. As with the SLC array 400embodiment of FIG. 4, it is again noted that in multiple page per rowembodiments of the present invention, coupling interference can also becompensated multiple write operations out/pages out. However, again,this requires increased circuit complexity and additional inputbuffer/data latch space.

FIG. 5C details the threshold voltage ranges 560 and the adjustment ofthe programmed state threshold voltages 554, 556, and 558 of themulti-level memory cells (MLCs) 502 of the memory array 500 embodimentof FIG. 5A. In FIG. 5C, memory cells 502 are initially erased into theerased threshold voltage state 544 (logical state “11”, a slightlynegative voltage). When programmed the final programmed thresholdvoltages of a group of memory cells 302 containing two or more pages ofdata are altered to a threshold voltage in ranges 554, 556, and 558, tocompensate them, depending on whether a directly adjacent (ornon-directly adjacent) memory cell 502 of the current row 504 orfollowing row 506 is also to be programmed to one of the allowedprogrammed states in a following programming operation. The thresholdvoltage of the programmed cell is determined by consulting the AVthmatrix 570 of FIG. 5C, that utilizes the final programmed thresholdvoltages of the memory cell 502 and the adjacent memory cell todetermine the coupling compensated threshold voltage to program into thememory cell, compensating for the coupling interference and inadvertentcharge injection.

FIG. 6 illustrates another example of a programming operation in a NANDarchitecture non-volatile memory device 600 having multiple pages perrow utilizing coupling compensation in accordance with anotherembodiment of the present invention. In FIG. 6, the memory device 600contains an array 650 of non-volatile memory cells 602 (represented hereby the intersection of each bit line and word line) arranged in rows604, 606, 608 and columns 610, 612 and having two pages (a page on theeven bit lines 610 and a page on the odd bit lines 612) in each row 604,606, 608. Data to be written is latched into data latches 646 of thememory device 600 after being transferred in from the memory bus. Acurrent row of data is then transferred from the data latches 646 to thesense amplifiers 644. The row 604, 606, 608 and the page of the row tobe written are selected from the decoded row address (not shown) and theeven bit line 610 or odd bit line 612 coupled to each sense amplifier644. The next page of data to be written is also latched at this timeinto the data latches 646. The timing control 642 selects the timing ofthe sense amplifiers 644 and/or programming word line voltage deliveredby the word line driver 640 to program the selected memory cells 602 ofthe page of the memory array 650. In doing so, the timing control 642utilizes the next page data stored in the data latches 646 to select andcompensate the threshold voltage levels of the current page of data forthe coupling effects predicted from the programming of the next page ofdata.

The page address/programming sequence of the memory array embodiment 600of FIG. 6 again transitions in an alternating “S” pattern between theeven bit line 610 and the odd bit line 612 so that the memory cells ofthe next page to be sequentially addressed/programmed are alwaysdirectly adjacent to those of the previously addressed/programmed page(sequentially writing the pages of the row from the first page to thelast page and then reversing and writing the last to first page of thenext row, and so on); Page A 620 (Even BL 610, WLN 604), Page B 622 (OddBL 612, WLN 604), Page C 624 (Odd BL 612, WLN+1 606), Page D 626 (EvenBL 610, WLN+1 606). In compensating Page A 620, the latch data containedin both the latch 646 (2) directly coupled to the sense amplifier 644programming the cell and that of the latch 646 (1) programming the celladjacent to the immediate left is utilized. In compensating Page B 622,the latch data contained only in the latch 646 (2) directly coupled tothe sense amplifier 644 is utilized. In compensating Page C 624, thelatch data contained in both the latch 646 (2) directly coupled to thesense amplifier 644 and that of the latch 646 (3) adjacent to theimmediate right is utilized. In compensating Page D 624, the latch datacontained only in the latch 646 (2) directly coupled to the senseamplifier 644 is utilized.

As will be apparent to one skilled in the art, differing word linevoltages, compensated threshold voltages, and patterns can be set inembodiments of the present invention to compensate for a variety ofdifferent memory cell to memory cell coupling ratios, memory cellgeometry, word line characteristics, program speeds or interferencesusceptibilities.

FIG. 7 is an illustration of an exemplary memory module 700. Memorymodule 700 is illustrated as a memory card, although the conceptsdiscussed with reference to memory module 700 are applicable to othertypes of removable or portable memory, e.g., USB flash drives, and areintended to be within the scope of “memory module” as used herein. Inaddition, although one example form factor is depicted in FIG. 7, theseconcepts are applicable to other form factors as well.

In some embodiments, memory module 700 will include a housing 705 (asdepicted) to enclose one or more memory devices 710, though such ahousing is not essential to all devices or device applications. At leastone memory device 710 is a non-volatile memory including circuits of oradapted to perform elements of methods of the present invention. Wherepresent, the housing 705 includes one or more contacts 715 forcommunication with a host device. Examples of host devices includedigital cameras, digital recording and playback devices, PDAs, personalcomputers, memory card readers, interface hubs and the like. For someembodiments, the contacts 715 are in the form of a standardizedinterface. For example, with a USB flash drive, the contacts 715 mightbe in the form of a USB Type-A male connector. For some embodiments, thecontacts 715 are in the form of a semi-proprietary interface, such asmight be found on CompactFlash™ memory cards licensed by SanDiskCorporation, Memory Stick™ memory cards licensed by Sony Corporation, SDSecure Digital™ memory cards licensed by Toshiba Corporation and thelike. In general, however, contacts 715 provide an interface for passingcontrol, address and/or data signals between the memory module 700 and ahost having compatible receptors for the contacts 715.

The memory module 700 may optionally include additional circuitry 720which may be one or more integrated circuits and/or discrete components.For some embodiments, the additional circuitry 720 may include a memorycontroller for controlling access across multiple memory devices 710and/or for providing a translation layer between an external host and amemory device 710. For example, there may not be a one-to-onecorrespondence between the number of contacts 715 and a number of I/Oconnections to the one or more memory devices 710. Thus, a memorycontroller could selectively couple an I/O connection (not shown in FIG.7) of a memory device 710 to receive the appropriate signal at theappropriate I/O connection at the appropriate time or to provide theappropriate signal at the appropriate contact 715 at the appropriatetime. Similarly, the communication protocol between a host and thememory module 700 may be different than what is required for access of amemory device 710. A memory controller could then translate the commandsequences received from a host into the appropriate command sequences toachieve the desired access to the memory device 710. Such translationmay further include changes in signal voltage levels in addition tocommand sequences.

The additional circuitry 720 may further include functionality unrelatedto control of a memory device 710 such as logic functions as might beperformed by an ASIC (application specific integrated circuit). Also,the additional circuitry 720 may include circuitry to restrict read orwrite access to the memory module 700, such as password protection,biometrics or the like. The additional circuitry 720 may includecircuitry to indicate a status of the memory module 700. For example,the additional circuitry 720 may include functionality to determinewhether power is being supplied to the memory module 700 and whether thememory module 700 is currently being accessed, and to display anindication of its status, such as a solid light while powered and aflashing light while being accessed. The additional circuitry 720 mayfurther include passive devices, such as decoupling capacitors to helpregulate power requirements within the memory module 700.

It is noted that other programming operations and threshold voltagelevels for non-volatile memory device and array embodiments of thepresent invention are possible and will be apparent for those skilled inthe art with the benefit of this disclosure.

CONCLUSION

A non-volatile memory device and programming process has been describedthat compensates for the capacitive coupling effects on programmedthreshold gate voltages of adjacent floating gate or non-conductivefloating node memory cells by adjusting the threshold voltage levelprogrammed into a memory cell in view of the coupling ratio and thedata/threshold value being programmed on a following programming cycleinto an adjacent memory cell, so that the coupling effect between thecells results in the desired target threshold voltages for the cells. Inone embodiment of the present invention, memory cell to memory cellcoupling is compensated for by adjusting programming level of one ormore memory cells of a first page or row of a memory array ornon-volatile memory device to a higher or lower threshold verify targetvoltage given the data/programming level to be written to directlyadjacent memory cells of a second page, so that coupling between thedirectly adjacent memory cells of the first and second pages brings theone or more memory cells of first page to their final target programminglevel.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement that is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. Many adaptations ofthe invention will be apparent to those of ordinary skill in the art.Accordingly, this application is intended to cover any adaptations orvariations of the invention. It is manifestly intended that thisinvention be limited only by the following claims and equivalentsthereof.

1. A method of programming a selected memory cell of an array of memorycells to a desired programming state, the method comprising: adjustingthe desired programming state to an adjusted programming state, whereinthe adjusted programming state comprises a programming state where theselected memory cell achieves the desired programming state in responseto coupling effects resulting from a subsequent programming operationperformed on a first memory cell adjacent to the selected memory cell.2. The method of claim 1, wherein a subsequent programming operationperformed on the first memory cell adjacent to the selected memory cellfurther comprises a subsequent programming operation performed on thefirst memory cell where the first memory cell is adjacent the selectedmemory cell by a row of the array of memory cells.
 3. The method ofclaim 1, wherein a subsequent programming operation performed on thefirst memory cell adjacent to the selected memory cell further comprisesa subsequent programming operation performed on the first memory cellwhere the first memory cell is adjacent the selected memory cell by acolumn of the array of memory cells.
 4. The method of claim 1, whereinadjusting the desired programming state to an adjusted programming statefurther comprises, adjusting the desired programming state to anadjusted programming state where the selected memory cell achieves thedesired programming state due to coupling effects resulting from asubsequent programming operation performed on a second memory celladjacent to the selected memory cell.
 5. The method of claim 4, furthercomprising programming the selected memory cell to the adjustedprogramming state prior to programming the second memory cell adjacentto the selected memory cell.
 6. The method of claim 5, wherein thesubsequent programming of the first memory cell and the second memorycells further comprises the subsequent programming of the first memorycell and the second memory cell as part of a programming operation of apage of memory cells.
 7. A method of programming memory cells of amemory array, the method comprising: receiving first data to be writtenin a first memory cell; receiving second data to be written in a secondmemory cell, wherein the second memory cell is adjacent the first memorycell and where the first and the second data each corresponds to one ofa plurality of programming levels to be programmed in the first and thesecond memory cells; determining a plurality of adjustment amounts,wherein each adjustment amount of the plurality of adjustment amountscorresponds to one of the plurality of programming levels; adjusting thefirst data by an adjustment amount corresponding to a programming levelof the received second data; and programming the first memory cell withthe adjusted first data prior to programming the second memory cell. 8.The method of claim 7, wherein determining a plurality of adjustmentamounts further comprises determining a plurality of adjustment amountswhere each adjustment amount compensates for coupling effects resultingfrom a programming operation of a particular programming level to beperformed on the second memory cell of the memory array.
 9. The methodof claim 7, wherein adjusting the first data further comprises adjustingthe first data by one of the plurality of adjustment amounts such thatthe first memory cell achieves a programming level corresponding to thereceived first data upon completion of subsequently programming thesecond memory cell with the second data.
 10. A method of compensatingfor programming interference effects in a memory device comprising anarray of memory cells, the method comprising: characterizing a firstcoupling ratio between a first plurality of memory cells and a secondplurality of memory cells of the array of memory cells where the firstplurality of memory cells are adjacent the second plurality of memorycells in the array of memory cells; receiving first data to beprogrammed into the first plurality of memory cells; receiving seconddata to be programmed into the second plurality of memory cells;adjusting the first data to be programmed into each of the firstplurality of memory cells by an amount dependent upon the second data tobe programmed into an adjacent memory cell of the second plurality ofmemory cells and further dependent upon the first characterized couplingratio; and programming the first plurality of memory cells with theadjusted first data prior to programming the second plurality of memorycells.
 11. The method of claim 10, further comprising characterizing asecond coupling ratio between the first plurality of memory cells and athird plurality of memory cells of the array where the first pluralityof memory cells are adjacent the third plurality of memory cells in thearray of memory cells, and receiving third data to be programmed intothe third plurality of memory cells.
 12. The method of claim 11, whereinadjusting the first data to be programmed into each of the firstplurality of memory cells further comprises adjusting the first data tobe programmed into each of the first plurality of memory cells whereadjusting the first data is further dependent upon the secondcharacterized coupling ratio and the third data to be programmed in eachadjacent memory cell of the third plurality of memory cells.
 13. Themethod of claim 12, wherein programming the first plurality of memorycells with the adjusted first data further comprises programming thefirst plurality of memory cells with the adjusted first data prior toprogramming the third plurality of memory cells.
 14. The method of claim10, wherein characterizing a first coupling ratio between a firstplurality of memory cells and a second plurality of memory cells of thearray of memory cells further comprises characterizing a first couplingratio between a first plurality of memory cells and a second pluralityof memory cells of the array of memory cells during manufacturing of thememory device.
 15. The method of claim 10, wherein characterizing afirst coupling ratio between a first plurality of memory cells and asecond plurality of memory cells further comprises characterizing afirst coupling ratio between a first plurality of memory cells and asecond plurality of memory cells where the first and the secondpluralities of memory cells comprise multi-level memory cells.
 16. Amethod of programming memory cells of an array of memory cells, themethod comprising: receiving a first data value to be programmed into afirst memory cell, the first data value corresponding to a first rangeof threshold voltages; receiving a second data value to be programmedinto a second memory cell, the second data value having a correspondingrange of threshold voltages, and where the second memory cell isadjacent to the first memory cell; and programming the first memory cellto a first adjusted threshold voltage outside of the first range ofthreshold voltages to compensate for interference by the subsequentprogramming of the second data value into the second memory cell. 17.The method of claim 16, further comprising receiving a third data valueto be programmed into a third memory cell the third data value having acorresponding range of threshold voltages, and where the third memorycell is adjacent to the second memory cell.
 18. The method of claim 17,further comprising programming the second memory cell to a secondadjusted threshold voltage outside of the second range of thresholdvoltages to compensate for interference by the subsequent programming ofthe third data value into the third memory cell.
 19. The method of claim18, further comprising receiving a fourth data value to be programmedinto a fourth memory cell the fourth data value having a correspondingrange of threshold voltages, and where the fourth memory cell isadjacent to the first and the third memory cells.
 20. The method ofclaim 19, further comprising programming the third memory cell to athird adjusted threshold voltage outside of the third range of thresholdvoltages to compensate for interference by the subsequent programming ofthe fourth data value into the fourth memory cell.